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Space Telescope Science Institute
NICMOS Coronagraphic and Grism Surveys for Brown Dwarfs

Don McCarthy
Steward Observatory, Univ. of Arizona
Tucson AZ

NICMOS, the Near-Infrared Camera and Multi-Object Spectrometer, is a near-infrared (0.8 to 2.5 micron) imager and low resolution (R~200) spectrometer scheduled for installation in the Hubble Space Telescope in February, 1997. Operating from orbit, NICMOS avoids the atmospheric effects (wavefront distortion, OH emission, and absorption) which limit the capabilities of ground-based instruments. Thus, NICMOS provides very stable, diffraction-limited resolution (~0.1 arcsec) with high dynamic range (exceeding several million using non-destructive reads). These capabilities enable the detection of low-luminosity brown dwarfs as close companions to nearby stars. Coronagraphic optics will suppress the direct and diffracted light from the primary star and enable companions to be detected outside of a 0.3 arcsec radius. The Science Team will conduct coronagraphic surveys of both nearby M dwarfs and young stars to spot orbiting brown dwarfs with masses as low as ~5 Jupiters. We discuss the observation strategies and capabilities of this technique. We will also search for brown dwarfs as isolated field objects by conducting a grism (1 to 2 micron) spectroscopic survey using NICMOS in a parallel mode of observation. This spectral region contains important diagnostic features of brown dwarfs, such as methane absorption. The sensitivity of NICMOS in this grism mode enables brown dwarfs like Gl 229B to be detected ten times farther away, and greatly exceeds the capabilities of any planned ground-based program. This survey should also detect and characterize many objects in the poorly understood transitional region between red- and brown dwarfs.